Paint an Object (I know this sounds ridiculous)
First, you want to go to the texture paint tab. Press new image and name it something so you could recognize it.
Go to the shading tab and assign an image texture to base color. In the dropdown, select your created paint image.

Now the image is assigned to the object. It should be black by default. Go back to the texture paint tab and you should be able to paint now. Press n to change color, size, and strength. Hope this helps!